1. Field of the Invention
The present. invention relates to an image signal processing apparatus and method, and a program. More specifically, the present invention relates to an image signal processing apparatus and method, and a program in which an image with appropriately reduced noise can be obtained even when gradation correction processing and noise reduction processing are performed in combination.
The present invention relates to an image signal processing apparatus and method, and a program. More specifically, the present invention relates to an image signal processing apparatus and method, and a program in which an image with appropriately reduced noise can be obtained even when gradation correction processing and noise reduction processing are performed in combination.
2. Description of the Related Art
With the recent advancement of solid-state imaging technologies such as noise reduction or advancement of signal processing technologies, even digital cameras have been provided with capabilities to capture image data having a wide dynamic range. In this case, exposure control is performed so as to reduce the amount of exposure compared to that for image capture under normal conditions in order to prevent saturation from occurring in imaging elements (see, for example, Japanese Unexamined Patent Application Publication No. 2008-148180).
Imaging that is performed in the above control is hereinafter referred to as “wide DR imaging” and an image that is captured under the above control is hereinafter referred to as a “wide DR image”.
Wide DR images are uniformly darker than images captured under normal conditions. Thus, when a wide DR image is output, inside a camera, gradation correction processing is performed in digital signal processing so as to optimize the brightness of the main subject. Examples of the gradation correction processing generally include tone curve processing and dynamic range compression processing.
The gradation correction processing is performed in such a manner that the image of the main subject can appear brighter. This operation has the effect of increasing the width of the signal which is assigned to the main subject. In the digital signal processing, therefore, signal continuity (the smoothness of change in level) for the corresponding portion is deteriorated.
In digital cameras, noise reduction processing (hereinafter referred to as “NR processing”) is generally performed in order to reduce noise in image data. In the NR processing, a noise component is removed from image data in consideration of a noise model indicating the relationship between the input level and the noise component (see, for example, Japanese Unexamined Patent Application Publication No. 2008-131529) or human visual characteristics (see, for example, Japanese Unexamined Patent Application Publication No. 2008-113222).
The NR processing is generally performed using, for example, a non-linear smoothing filter such as a bilateral filter. The process for smoothing a digital signal has the effect of improving the continuity (resolution) in level by averaging a plurality of pieces of data including quantization error and noise. Therefore, the NR processing also has a nature of improved signal continuity of an input image as well as noise elimination or reduction.